The researchers said their finding is surprising and alarming because the drug plays a major role in treating and preventing infection with HIV, the virus that causes AIDS.

"Tenofovir is a critical part of our armamentarium against HIV, so it is extremely concerning to see such a high level of resistance to this drug," study author Dr. Ravi Gupta, from the department of infection and immunity at University College London in England, said in a university news release.

"It is very potent drug with few side effects, and there aren't any good alternatives that can be deployed using a public health approach. Tenofovir is used not only to treat HIV but also to prevent it in high-risk groups, so we urgently need to do more to combat the problem of emerging resistance," Gupta said.

Resistance often occurs when patients don't take their drugs as directed. To prevent resistance, people need to take the drugs correctly about 85 percent to 90 percent of the time, the researchers said.

For the study, the investigators looked at more than 1,900 HIV patients worldwide who had uncontrolled HIV despite taking antiretroviral drugs. Tenofovir-resistant HIV strains were found in 60 percent of patients in sub-Sahara Africa, the researchers found. That compares to just 20 percent of patients in Europe with tenofovir-resistant strains, the researchers said.

About-two thirds of patients with tenofovir-resistant HIV also had resistance to both other drugs used in their therapy. This suggests that their treatment was totally compromised, the study authors said.

In sub-Sahara Africa, up to 15 percent of HIV patients treated with tenofovir-based drug combinations will develop resistance to tenofovir in the first year of treatment, and this rate is likely to rise over time, the researchers estimated.

They added that tenofovir-resistant HIV strains could be passed on to other people and become more widespread, potentially weakening global efforts to control HIV.

It's not clear how likely drug-resistant strains of HIV are to spread. If these strains were less effective at spreading, Gupta said the researchers should've seen lower levels of the HIV virus in people with the resistant strain. But, that wasn't the case.

"We found that virus levels were no lower in individuals with the resistant strain and were high enough to be fully infectious. We certainly cannot dismiss the possibility that resistant strains can spread between people and should not be complacent. We are now conducting further studies to get a more detailed picture of how tenofovir resistant viruses develop and spread," he concluded.